1. Field of the Invention
The present invention relates to an electroacoustic transducer that emits sound waves into a liquid, and in particular, an acoustic transducer that can efficiently emit sound waves underwater.
2. Description of Related Art
As an acoustic transducer including a ring oscillator that emits sound waves into a liquid such as water, there is known a transducer provided with one or more annular members in the axial direction, which are formed such that the interior is hollow. This acoustic transducer, by turning on an electric current to the annular member and oscillating the entire ring oscillator, emits sound waves based on the oscillation into the liquid.
Examples of acoustic transducers include those disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-333487 (hereunder, referred to as Patent Document 1), and Japanese Unexamined Patent Application, First Publication No. S60-196100 (hereunder, referred to as Patent Document 2).
The acoustic transducer disclosed in Patent Document 1 has a configuration in which a plurality of ring oscillators formed with a hollow interior are coaxially and consecutively provided in the axial direction, the axial direction upper end portion is supported by a ring, and the axial direction lower end portion is supported by a ring. Furthermore, the ring on the axial direction upper end portion and the ring on the axial direction lower end portion are fixed by means of a bolt, which penetrates through the hollow interior, and a nut. Moreover, on the periphery of the ring oscillator, a thin-walled axial direction diaphragm that vibrates in a bending vibrational mode is supported, and disk-shaped end face diaphragms that vibrate in a bending vibrational mode are respectively provided on both end portions of this axial direction diaphragm. Furthermore, broadband acoustic emission characteristics are able to be obtained by making the mechanical resonance frequencies of the axial direction diaphragm and the end face diaphragms different.
As a similar construction, a method has been proposed in which a plurality of thin-walled rectangular shaped diaphragms that are longer in the axial direction, and to which a piezoelectric oscillator that vibrates in a bending vibrational mode is attached, are arranged on the circumference, and the bending vibrations are utilized to perform acoustic emission underwater.
The acoustic transducer disclosed in Patent Document 2 includes a coil-shaped oscillator that is formed in a spiral shape by an electrostrictive material that generates distortions according to an applied voltage. Furthermore, the upper end portion and the lower end portion of the coil-shaped oscillator are fixed by a metal fitting. As a result, it is made possible for the vibrations generated in the coil-shaped oscillator to be converted into low-frequency radial direction vibrations according to the length of the coil-shaped oscillating body.
In acoustic transducers that perform acoustic emissions underwater by using the breathing vibrations of the ring oscillator, in the case of a construction in which both ends are sealed such that liquid does not flow in, and the interior is further filled with air or the like, there is a problem in that if it is driven at below the frequency of the resonance frequency of the breathing vibration mode of the ring oscillator, a highly efficient acoustic emission can not be obtained.
As the driving force that is generally utilized as the ring oscillator, ring-type piezoelectric oscillators such as piezoelectric ceramics, in which the construction and driving is simple, or polygon-shaped ring oscillators in which rectangular piezoelectric oscillators are aligned and laminated in a cylindrical shape, are used. Lead zirconate titanate serving as the piezoelectric material, is a material in which the mass and elasticity modulus are virtually the same as metals.
The resonance frequency of the ring oscillator is proportional to the square root of the elasticity modulus of the constituent material, and inversely proportional to the square root of the density, of the constituent material. In a case where the circumference length of the ring oscillator corresponds to one wavelength of the speed of sound of the constituent material, the ring oscillator vibrates in a breathing vibrational mode in which it uniformly fluctuates from a basic state, to a contracted state with a small diameter, and to an expanded state with a large diameter. This resonance frequency is very high because, as mentioned above, it is determined by the density and the elasticity modulus of the piezoelectric ceramic, and the speed of sound of the piezoelectric ceramic is approximately as fast as a metal.
Although there are piezoelectric ceramics referred to as a soft-type, which have a small elasticity modulus, it is not possible to greatly lower the resonance frequency.
In general, as the ring oscillator has a construction in which electrodes are arranged on the inner and outer faces, breathing vibrations of the ring oscillator are excited by a piezoelectric transverse effect, and acoustic emission is performed from the outside of the ring oscillator. Furthermore, it is common for the ring oscillator to be coated with a sheath or to be molded by means of a synthetic resin for protection, such that short-circuiting does not occur owing to the surrounding liquid.
End plates are arranged on the end faces of the ring oscillator, such that the liquid does not enter into the interior of the ring oscillator. Furthermore, a cushioning material, such as cork or laminated paper, is provided between the ring oscillator and the end plates, such that the end plates do not inhibit the breathing vibrations of the ring oscillator.
In order that the surrounding liquid does not enter into the interior from the gap between the end plates and the ring oscillator, this portion is also furnished with a sheath or a mold.
As another method, a ring oscillator is configured to approach a cylindrical shape by aligning rectangular piezoelectric oscillators which have an approximately rectangular parallelepiped shape in a polygonal shape via a wedge block having an approximately trigonal prism shape.
The acoustic transducers of the constructions mentioned above are able to perform acoustic emissions most efficiently at the time of the breathing vibrational mode, which occurs in a case where the cylinder length of the ring oscillator corresponds to one wavelength of the speed of sound of the constitution material.
According to common piezoelectric ceramic materials, in the case of a cylinder of a radius of approximately 10 cm, the resonance frequency of the breathing vibrational mode is approximately 5 to 10 kHz. In a case where it is utilized at below this frequency, since it deviates from the resonance frequency of the ring oscillator, the acoustic emission efficiency thereof will inevitably become low.
On the other hand, in a free-flooded type acoustic transducer in which end plates are not provided and water is also introduced into the interior of the ring oscillator, by utilizing the resonance of the breathing vibrational mode of the ring oscillator and the resonance of the water in the ring oscillator interior (water column resonance), acoustic emission with good efficiency can be performed.
In a case where the water column resonance is utilized at a frequency below the resonance frequency of the breathing vibrational mode, the acoustic emission efficiency inevitably becomes low. In other words, in regard to the frequency of the water column resonance, the resonance frequency is determined in an inverse relation to the height of the cylindrical oscillating body. Therefore, the frequency of the water column resonance can be designed independently from the breathing vibrations. However, the water column resonance cannot be obtained with a good efficiency if the frequencies of the two are separated, because the driving force is the breathing vibrational modes.
Even if one of the vibrations among the breathing vibrational mode or the water column resonance is to be utilized, in order to be used efficiently at a lower frequency, it is necessary to make the diameter of the cylinder larger, and as such, larger dimensions and mass are required as a result.
As a conventional example, in a case where it is made a spiral shape construction such as that disclosed in Patent Document 2, it is possible to lengthen the total length of the coil-shaped oscillating body, and it is possible to reduce the resonance frequency in the length direction thereof. However flexural vibrations of the entire coil lower than the resonance frequency of the oscillating body longitudinal direction are generated as a result of the structural asymmetry of the oscillating body, and there is a problem in that the required vibrations in the coil longitudinal direction do not necessarily become radial direction expansions and contractions of the cylinder, or in other words, breathing vibrations.